ussnorway: p.s, Steam deck isn't a Gog thing but a Steam server thing... clue is in the name but yes you could hack it if you are willing to go back and rehack it again every single time Galaxy updates
I own the GOG version, so the Steam servers aren't involved at all. A Steam deck is just a portable Linux PC, there are multiple ways to install and run GOG games on it, no "hacking" required. You can directly install and run GOG Galaxy, which is made very simple with the
NonSteamLaunchers script, or use a third party launcher like
Heroic, amongst other options.
For Larian cross saves to work, the only requirement is that the game is launched via LariLauncher.exe - GOG Galaxy is not necessary, as the only things it provides for Baldurs Gate 3 are updates and achievement tracking.